


Proof of Concept

by jendavis



Series: Sam and Bucky Endgame Post Game Fics [1]
Category: The Avengers (Marvel Movies)
Genre: Fix-It, Headcanon, I Wrote This Instead of Sleeping, M/M, endgame spoilers, no beta we die like men
Language: English
Status: Completed
Published: 2019-04-30
Updated: 2019-04-30
Packaged: 2020-02-10 06:00:09
Rating: General Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,499
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/18654361
Author URL: https://archiveofourown.org/users/jendavis/pseuds/jendavis
Summary: How it should've ended.(again, spoilers!)





	Proof of Concept

Bucky's never been a general. He's never been the one giving the orders. But he knows strategy.

He knows how to take out an enemy targets on a crowded battlefield, clearing a path for the heavy hitters to get through. He knows how to duck below the screaming line of Steve's shield, how to swing around and provide cover fire because the self-sacrificing idiot's just left himself _open, again_. 

At least _Bucky_ knows how to cover his own ass. Not so long ago, even going out for something as mundane as a _grocery run_ had to be planned six ways from Sunday. He mapped escape routes in his head and scraped together safehouses from nothing at all, because _someone_ was always closing in. Even when he'd been without any self to speak of, the _asset_ had been able to could condense five-word orders into completed missions -one dead target, half a dozen, a medium-sized Embassy, it hadn't mattered- and get himself out to the extraction point, unseen and without backup. 

Before all that, though, he'd first had to learn how to back up his idiot runt of a best friend who never missed an opportunity to pick a fight with bullies twice his size. Sometimes it was dragging them off of Steve, sometimes it was the other way around. Sometimes it was deciding to risk being glared at from shoulder height for hours, just for heading the whole mess off with a joke and a smile.

And while he might not've ever figured out how to talk Steve into- or out of- anything, he does know how Steve thinks. Which is why he knows, even before he asks, what Steve's planning to do. And why he's not surprised when he doesn't get a full answer. 

\---

"Catch you guys in a few," Sam nods, and starts making his way back over to Pepper to pay his respects. He's moving easy, like it's not something he even needs to square his shoulders for. Bucky's tried to approach the patio twice, and been relieved, twice, that someone else had gotten there first. 

"You good, Buck?" 

Steve's always asking that, these days, like some kind of revenge for the first two decades of their acquaintance. "Yeah."

"You sure? You're kind of zoning out there." The accusation's quiet and when he turns to look, the smirk's low key, appropriately unobtrusive for the post-funeral mood.

It's not the first time for that, either. "Fuck off."

"No. I mean, it's good. Sam's a good guy."

Bucky glares off into the trees, scanning for any movement that shouldn't be there, not caring if it's merely deliberate or habit. Because yeah, he _knows_. Sam's an _infuriatingly_ good guy, and enough of an asshole that it isn't suffocating. Even if sometimes, more and more lately, he can't exactly breathe around him. "He's got more sense than you, anyway."

"You're not wrong," he says, turning to watch Fury approach. "Keep an eye on him while I'm gone, all right?"

"Yeah." It occurs to him, belatedly, that he doesn't know how any of this time travel stuff works. Banner'd said Steve would only be gone a few seconds; Bucky's not sure what else to ask, only that someone probably _should_.

Because just a few days ago, he'd felt his limbs disintegrating- not painful, just confusing- and then there'd been _something_ that he can't remember. Something alien and _wrong_ or maybe just new. Then- a minute or a day or a decade later- the feeling, almost like being thawed- only suddenly and all at once and without lab coats staring at him under thin blue lights. There'd been tense voices shouting through the trees. 

He'd found Sam, first, eyes wide and wary and just as lost as him.

Billions of people had, in that moment, been having the same _what the fuck_ conversation, but he and Sam, sticking close together as they joined Shuri, T'Challa, and the hundreds of others pouring back out onto the empty Wakandan battlefield, had only a few moments to register the lack of fallen bodies before being called back into the fight. There hadn't been time to dwell. And that, in retrospect, had probably been for the best. 

Finding out that they'd all lost five years hadn't come until afterwards. 

The fighting had stopped- Stark had saved them all, somehow- the reunions had started. 

It'd rolled off Bucky more easily than most; it didn't mean he'd known how to help. Sam'd been trying to, for everyone they came across. He'd been bruised, beaten up, physically exhausted, and the shock had finally started setting in. In the absence of any kind of plan, Bucky reverted to the usual strategy: stay close and ensure that this particular instance of _I-could-do-this-all-day_ stubbornness doesn't have him face-planting into torn up concrete. 

Back in Wakanda and fresh out of stasis, Bucky'd only played along with the Sam's exercises out of spite. Sitting there in the rubble, dead and injured all over, he'd done such a bad job trying to talk Sam through them that he'd actually gotten laughed at for his troubles. 

But at least Sam hadn't fought him on just _sitting down to breathe_ for a minute, which is more than he could say for a lot of people. 

\--- 

_Until the end of the line, that was the fucking deal_ , he wants to tell Steve, two days after the funeral. 

“I’ll miss you,” he says instead, because whatever he's doing here, Bucky's not going to try fighting him on it. _That_ tactic is reserved for his bloody-minded, self-sacrificing moments, and this, here, looks like genuine hope. Like peace, and he doesn't think he's really seen it since they were kids. 

Steve grins, his eyes confirming everything but the details. “Try not to do anything stupid while I’m gone.” 

“How can I? You're taking all the stupid with you.” 

And Bucky can't follow, can't help, where he's going.

\---

The silhouette's there, on the bench looking out over the lake, looking so much like Steve, lifetimes ago, sitting on the fire escape with that beat up portfolio that would have him staggering to hold onto it whenever the wind blew hard, that it's almost startling when he realizes it _is_ him. 

It takes Sam another few seconds to figure it out. 

Bucky sends him up ahead- _this_ much of the plan, Steve'd let out of the bag, twenty minutes- and for Steve, several decades- ago. 

It gives him a second to freak the fuck out without anyone watching. 

He hasn't, so far as he can tell, flashed instantly into nothingness any time in the past few minutes. He'd heard Banner talking about how it's a myth, but that's probably more reassuring to someone whose very existence hadn't come down, just a few days ago, to the snap of someone's fingers. 

Billions of people had blinked, and five years had passed. If that's not time travel, Bucky's not sure what is. But apparently, with this, the prevailing theory still holds. The grass and soil give slightly when he digs his heel in, but the ground is otherwise solid. He's still here. 

And he's still _himself_. Nothing's different, nothing's changed. 

They're all still here, they're all alright. And right now, up ahead, Steve's passing Sam the shield to make sure they _will_ be. 

And there's this weird angry twist in his gut that he can't identify, because Steve's never been in the middle of it like this. 

He tries breaking it down and picking it apart, the way Sam told him to. 

It's not about the shield. Bucky'd killed the man who'd made it, and Steve'd had to use it against Bucky. That Steve was his friend first feels a bit more accurate, but acknowledging it doesn't lend any sort of clarity, it just muddles it more. _Closer_ , but it's still not it. 

Up ahead, Sam's waving him over with the arm that isn't holding the shield like he's not quite sure what to do with it. And suddenly, Bucky feels like he's in one of those cartoons, like he's run off a cliff and just hasn't looked down, yet. Willing his feet to move, Sam meets him halfway.

"You guys catch up," he says, catching him by the shoulder and squeezing lightly. "I'll be up at the house when you're done."

Relieved by the order- by the hint that at some point in the near future, he'll have that much of a mission at least- he nods. He doesn't move on until Sam does. 

\--- 

The shape of Steve is different, but he's starting to get used to that. He still feels more solid, though, than he had when they were kids. And Bucky might be totally at sea, here, but he can't help laughing when he sees the ring. 

"You finally get that dance?"

Steve beams, nodding as he eases himself back down onto the bench. "Fifty eight years. Peggy says hi."

Sitting down, he doesn't know what to say to that. 

"You look good," Steve continues, shaking his head. "Sorry. I know, this is weird. It's just _really_ good to see you." 

With the new lines to his face, it takes a moment to realize that he's blinking back tears. Suddenly alert, Bucky draws himself up, looks over at him to find that he can still recognize regret in eyes that heavily lined. "Steve?"

"I'm sorry, Buck." He takes a breath. "Just. Haven't seen you in fifty years. And I can't stick around now, either. Promised Peggy I'd be home for dinner."

"So why'd you come back at all? And why'd it take so long?"

"I couldn't leave everyone wondering if it had worked. But I couldn't leave _there_ without knowing for sure I'd be able to get back home."

"But you figured it out?"

"Tony and Hank Pym, eventually." He smiles. "Advantage of foresight."

He can't help the bitterness in his voice, and doesn't really try to. "Sounds nice."

"Yeah." Steve nods to himself, then leans down to the bag at his feet, pulling out a metal box. "Just in case it takes a while to get another batch going." Nodding at the time machine platform behind them, he hands it over. The box is filled with foam-packed red capsules; Steve nods back at the time machine platform without even having to guess it's direction. "Seems a waste, all that engineering, just rusting away. You want to hang onto them for me?"

"Thinking I should pass these on to Bruce."

The conspiratorial grin has him wanting, absurdly, to laugh. "I'm sure you all can come to some kind of arrangement. But Peggy wants to meet you, if you want to come by, when you get the chance." 

"Sure thing," he says, finally identifying what's been throwing him off. "You said fifty years. I'm confused."

"You're talking to a guy who was in 2004 just a few minutes ago, and now I'm in my past and my future at the same time. But yeah. Went back to '45. Lived my life for a while, kept my hand in where I could." His face goes slack, and his eyes are haunted when he looks up. "I went looking. Couldn't help it. Found you in 1973. I mean, I know it's not _you_ you, but." he breaks off with a sharp, angry frown. "Couldn't just leave you like that. Had to at least try. But it went bad. We managed to bring you. _Him_ in. I thought we could get around the programming, get him free. But it was too strong, nothing worked." He lets out a breath, shoulders slumping. "I went looking for a shrink- had a line on one in Dayton. But when I was gone, you were executed in secret. I didn't even find out about it until a week later." 

His stomach twists again, the half-identified anger boiling over again and then, suddenly, fading. He wants to ask why they'd killed him. He knows he would've deserved it. It was him, it _wasn't_ him. Alternate timelines, plural, like there might be others. He should probably be dead in most of them, by now. 

He leans over, knocking their shoulders together. "Hey, at least you tried." 

\--- 

He sticks around, listens to Steve talk about his life, his world, his reality and the shape of it. Clearing Hydra out of the SHIELD ranks in the mid-1960s. Fury pulling the Avengers together in the 1990s. New York and Sokovia had never fallen, but in his timeline, they're preparing for Thanos, all the same. They think they can head him off; Bucky hopes Steve's right. That's about all he's going to be able to do, from here. 

Steve's going to be leaving again- for good, this time- in a little less than an hour. This timeline hasn't been his home in decades. He's got people- a family- to get back to. 

Banner and the Langs are already back at work recalibrating the machine. Sam had disappeared half an hour ago. And he should stick around here, but the cabin's atmosphere is almost as stifling as it had been the morning of the funeral. Everyone'd been ready to head back into town, giving the family their space, but Steve's return- planned and unplanned as it had been- has thrown a wrench into the workings. 

He knows he should stick around. He thinks he might break something if he does, so he slips out through the kitchen, down the patio steps, and starts wandering, telling himself he's just looking for Sam to give him an update. Wouldn't be right, him missing the big final send-off

He finds him out on the other side of the ridiculously oversized garage. He's sitting with his back against the wall, the shield braced against his knees, steadied with one finger from each hand, eyes a thousand miles away.

"How're you holdin' up?"

"Me?" Sam startles. "I'm fine. Just thinking."

"You're holding that thing like it's about to explode."

"Just because it hasn't doesn't mean it won't."

"I hear you." Bucky leans against the tree, a few feet away. "It's bullshit, but I get it."

That, at least, gets a laugh. "Actually, you've been around since this was the height of technology, do you know how he throws it? Makes it come back?"

He shakes his head, shrugging. "Still got a while to go and ask him, if you want."

"Everyone still talking about this other timeline?"

"Yeah."

Sam looks up at him, then back at the shield, shaking his head with a smirk. "I'm good."

That's the best plan he's heard all day. Bucky sits down. Just barely within kicking distance, not too close. 

"Hey, man. I gotta ask. You good with this?" The shield rocks forward on his knees a bit, glinting in the late afternoon sun. 

"Weird to see someone with half a brain holding it, but yeah." He thinks about saying that he might've actually told Steve that he'd thought so, hours and years ago, but he's not sure and it might be presumptive. "Think you can pull it off with the wings?"

"Weight's more of an issue than drag, probably," Sam says, once he's paused to consider it. "Just don't know how I'm supposed to fight with just one arm. Got any pointers?"

Bucky flips him off with one silver finger, and Sam grins, setting the shield aside. He doesn't get up, though. Just looks at him. "So what's up with you? How'd your talk go?"

"Fine. Weird." He picks at a piece of grass, twisting it until it breaks. "Said he tried getting that Bucky out."

"Out?" Sam says, then his eyes wide. "Oh."

Bucky nods, shrugs. Bucky'd been killed, in secret, in 1978, and it's not Steve's fault. Hell, maybe _that_ Bucky would've thanked him for it, if he'd had half the chance to _be_ Bucky long enough to entertain the thought. 

"So. _Tried_ ," Sam says, doing that thing where he locks onto shit you'd rather he didn't. "I'm guessing it didn't take?"

"Got me out. Think SHIELD had me executed, not really sure. Steve wasn't there," he's quick to clarify. 

"Still. Sorry to hear that."

 _It's all right_ , he wants to say, but the words get stuck and if he says one thing, it'll lead to another, and another, and he's not up for it right now. "other than that, seems like everything's goin' pretty well, though. So that's good."

"You know, all these alternate timelines," Sam rocks his head to look at him. "Could be one or two where he pulled you out."

It's a nice thought, almost. But he's barely wrapped his head around Steve having an entire lifetime play out like he wanted; contemplating anything like that for himself just seems a bridge too far. "Yeah, well. Could be one or two where you're not sittin' out here looking like someone just handed you a death sentence."

"Could be," Sam shoots back, "long as there's a version somewhere where I'm running around, all shot up with that serum, with Steve's cloned brain in my head."

"Maybe you just worry less about throwing the shield around and maybe try using it _as_ a shield. That would be novel." Bucky suggests, more honestly than he means to. Because yeah, gods and monsters and all that, and Sam's not _just_ anything, but he's human. Too easily killed, and too good of a person to turn the offer down. 

"I dunno, man. Feels like I'd be punching above my grade, you know?"

Bucky actually laughs, because somewhere, in the past few minutes, he's realized- or maybe decided- two things. Sam's going to need someone on his six. And Bucky's already decided it's going to be him. "Pretty sure that's part of the deal." 

\--- 

There's voices out in the yard, up on the driveway, and this is it. In a few minutes, they'll be seeing Steve off for good. 

But once he's standing, his feet won't move. 

"Hey, I know," Sam says, picking up his shield by the rim. "This is gonna suck, but it'll only be a few minutes, yeah? Then we'll get the hell out of here. Go see if my house keys still work, try our luck at the base if they don't."

They'd talked about this on the drive out here, but it hadn't been settled, there'd been too much chaos to even think that far ahead. It's times like this, he worries if it's some latent thread of Hydra's leftover programming, how much of a relief it is to follow orders, to not have to strategize. 

"You sure?" 

"Course. Need you on this, man." Sam's eyes dart away, then back again, like he's backed into an alley, rolling his sleeves up for the fight. "And it don't make no sense without you. Deal?"

 _No_ , he realizes, _this_ programming was in place well before Hydra ever got their hands on him. 

"Deal." 

"My _man_!" They shake, and Sam wraps his arm around him fast enough that the shield clanks against his shoulder. 

He's hugged Steve before, he'd hugged Shuri. This one feels different, more balanced and precarious, all at once. 

\--- 

Steve's standing down by the lake again, and this time it's Bucky who goes first. 

"So," he claps Steve on the shoulder, not as heavily as he would've this morning. "This is the end of the line, huh?"

"Guess that depends on which line you're talking about." Steve turns to squint up at him. "And not if you come to dinner next Sunday."

Right. The Pym particles. "When's that?"

"First weekend of May, 2004. Church and East 37th, yellow house. Key's under the flower pot by the mail slot."

Time machines, and keys on front porches. It's surreal.

"All right, I'll be there." He hugs Steve, can't help comparing his arms and shoulders to Sam's. 

"And bring Sam along," Steve says, quietly smug, like he's caught him somehow.

"Steve."

"What?" Steve pulls back. "Not like I'm trying to match you with Darlene Jenkins. And, hey, time travel. Multiple dimensions. Proof of concept."

He stares, unmoving, at the implication, because if he turns even a fraction, everyone back up on the patio will be able to see how red his face must be getting. 

"You have all the time in the world to pull your head out of your ass. But if you both show up old and gray, you'll fit right in."

"Then maybe it's best that we wait," he tosses back. "Wouldn't want to shake up the senior center."

\--- 

Sam, when he's walking back from his turn, is wearing a mortified look that feels damningly familiar. He won't look him in the face, but Bucky can feel his eyes burning into the side of his head as they watch Steve climb up onto, and then vanish from, the time machine. When they finally make their way back to their vehicles, he barely says a word. 

Thing is, though, maybe, he _might_ look like he wants to, like he's trying to talk himself into it. 

For miles, Bucky just thinks about _old_ and _gray_ , and the odds of either of them making it that long. 

It's probably about seventy miles before they find some kind of even keel. 

\--- 

It's another two months before he works up the nerve to ask. 

The first month, there'd been no time. The world's population had suddenly doubled, there was shelter and infrastructure to do with. Billions of people needed to get shuffled back into society, and dozens who'd crawled out of the woodwork to make that harder. Any time not spent out in the field on one patrol or another, they'd face-planted into their beds along with the two dozen others in the barracks. 

The second month, he'd just been making excuses. Sam's housing situation had gotten straightened out- he'd lost the house, but had been cleared for an apartment- and for the first few weeks, while Sam set his gear up and began, seriously, to start thinking about how to best deploy this reality's Captain America, Bucky just tried to stay out of the way and not break anything. 

Not old or gray, yet, but not inevitable, either, even if he's content to turn the idea around in his head. 

And this week, Sam's gone quiet around him again, like sitting up half a night telling increasingly ridiculous stories about bad fights and incompetent CO's had used up his words. And it's not like Bucky's ever been particularly good at filling the void. 

So finally, he asks. "What did Steve tell you, when he left?"

Sam's eyes go wide, but he recovers quickly, twisting his fork into his spaghetti. "Just. You know, tried hyping me up on the Captain front. How important it was, getting everyone to work together." He's spun too much onto his fork, and tries again, looking anxious enough that Bucky wishes he'd just kept his damned mouth shut. "Said some shit, too, about alternate timelines." The anxiety is, apparently, contagious. But Sam doesn't make him ask. "Nothing tactical, just... more like, the kind of thing you don't want to repeat 'cause you'd always be wondering if things fell into place because they were real, or because you just wanted to go along and play along like they were."

Bucky's face and neck feel hot, so he looks down at his plate. Hopes like hell the hair's providing some cover. 

Out of the corner of his eye, he can see the shield, propped up against the wall next to the front door. 

"Think he pretty much told me the same thing," he admits to his plate. "Kind of think he might've been telling me things I already knew." 

\--- 

He doesn't work up the nerve to kiss him until a week later, when _old and gray_ suddenly looks like a distinct impossibility. 

"Told you not to _throw_ the damned thing," he says, easing his feet over the edge of the open helicopter door, dangling them a foot off the pavement. 

"Says the man who'd be filled with even _more_ bullets if I hadn't," Sam bites back, stepping sharply out of the doorway to let the nervous medic climb down out of the helicopter, clearly eager to give them some space. Bucky can practically see Sam count to ten before he rounds back. "You scared the hell out of me, man, you were going _down_ -"

Sam's angry and wired, and Bucky's suddenly starting to get it, and if they don't get ahead of it, soon, the doctors are going to be coming back to discover Captain America in a complete fucking meltdown. Hurriedly, he eases his jacket over the bandages. 

"Hey, shit, no," Bucky says, reaching for him, the pointless stitches stretching his good shoulder. "C'mere."

Sighing heavily, Sam does. Steps close enough that Bucky's knee hits him in the hip when he shifts. By then, Bucky's got his arms around him. It's a little awkward, a little exposed, so he keeps his voice low. 

"I'm sorry. I'm okay, though. Really."

"Not for lack of trying," Sam grumbles, but he lets himself be eased in. Manages to relax against him, except for where his fist is gripping tightly at the unzipped corner of his jacket. 

"You gotta let me draw fire sometimes. Bullets, I can take." He leaves the rest hanging, because this is still new, because this is new and they're _outside_ and someone could come around the side of the helicopter any moment. Fucking _Lang_ , probably, with the luck they've been having today. 

"I don't want you to," Sam mutters, tugging on the leather. But he eases his head back to look up at him, eyes narrowed and searching. And he doesn't let go. 

"I know."

He _doesn't_ know who leans in first. Just that they're _already_ so close, and hell. Multiple directions. Proof of concept. The only strategy needed is not overbalancing when he leans in, and then Sam's mouth is crashing against his, the sweat on his neck is damp against Bucky's fingers. These, he catalogs against the stillness in his lungs and the pounding in his chest. 

It's brief, and a little pissed off, and it's diving through the ice after skating along the edges for too long already. 

Sam's grinning wide, even before he pulls away. 

"Don't think you're gonna get out of every tactical disagreement this way," he warns.

"Wouldn't dream of it." 

 

\--- 

It's another two months before they can get cleared for the time jump; there's a vehicle on the way to get them through security and onto the base. 

It'll be here any minute. Which means they need to get their toothbrushes packed away and finish looking for Sam's phone charger. But the couch is comfortable. Warm where the breeze coming in through the open window is not. 

"We should probably get going," Sam says, halfheartedly shoving at Bucky's arm, which he has draped over his side. Then giving up, again, and relaxing.

"Why?" Bucky asks, curling in closer. "I could do this all day."


End file.
